Deployable structures made for use in space have generally been linearly deployed systems extended from a fixed base. These structures are commonly referred to as booms and can be divided into two categories: shell and lattice booms. Successful designs share traits of mass efficiency, low packaged volume, and reliability.
One specific type of a shell boom, referred to as a STEM in the space industry, for Storable Tubular Extendable Member, is disclosed in U.S. Pat. No. 3,144,104. A STEM, which typically incorporates a coilable thin metal strip, such as shown in the figures of U.S. Pat. No. 3,144,104, are precurved and form a cylindrical shell when deploying. These devices have found use in low load applications such as antennas and gravity gradient stabilization booms due to their compactness.
In general, a cylindrical shell, or tube, is a simple and mass efficient structure. However, STEMs have strength limitations since the deployed metal strip does not form a closed section. Multiple overlapped STEMs, such as those shown in various figures of U.S. Pat. No. 3,434,674 and methods of interlocking the overlapped section(s) (see, e.g., FIG. 8 of U.S. Pat. No. 3,144,104) have been pursued to increase strength.
STEMs are also limited in utility due to susceptibility to thermally induced bending. In space applications, one side of the STEM can be heated by solar radiation while the other side of the STEM would be shielded from solar radiation by the first side of the STEM. Such inconsistent heating could cause the side of the STEM receiving the solar radiation to expand more than the shielded side of the STEM, resulting in thermal induced bending, and distortion, of the STEM. Distortions in booms are generally undesirable and, in particular, distortions can reduce axial load capability.
A boom loaded axially, as a column, must be very straight to obtain full load capability. In practice, therefore, the ratio of the length to diameter of a boom is generally kept below the point where factors such as manufacturing straightness errors, thermal bending, and unintended minor load eccentricities leave the boom susceptible to collapse by buckling. While such factors will generally depend, at least in part, on the material system used to fabricate the boom, typically the length to diameter ratio of booms that are loaded as columns is maintained well below 80.
For long booms with moderate load and/or higher stiffness requirements, larger diameters are required to limit the slenderness ratio. However, using a boom comprised of a simple thin-walled cylinder will not be mass optimized, if the required wall thickness of the boom is greater than the optimal wall thickness required to meet stiffness requirements. This situation occurs when the optimal wall thickness required to meet stiffness concerns is so thin that the boom wall would have unreliable strength properties due to small manufacturing imperfections in the boom wall. Such imperfections are difficult to avoid and to predict. In addition, as optimum wall thickness decreases, such imperfections are more likely to initiate failure by local wall buckling from loads well below the bulk properties of the material being used to fabricate the boom. This limits the usefulness of thin-walled tubular shell booms, such as STEMs, and is one impetus for using a range of lattice type boom structures in space applications.
A lattice boom typically comprises a number of axially arranged structural elements, which are frequently referred to as longerons. Typically, the longerons are braced in a repeating fashion at intervals often referred to as bays. The longerons are typically rods, or sometimes tubes, and are braced at close intervals to prevent slenderness concerns from arising at the bay level. Structural members used to perform the bracing function are often referred to as battens. Diagonals, or as also sometimes referred to as stays or cross-members, are typically provided along each face of the bays to add structural rigidity. Diagonals, for example, may be in the form of crossing cables, each bearing tension only, or one or more rigid structural members capable of bearing both tension and compression.
One bay of a typical collapsible four-sided lattice structure is shown in FIG. 1 of U.S. Pat. No. 5,016,418, issuing to Rhodes et al., the disclosure of which is hereby incorporated by reference. Each bay, or structural unit, is constructed of structural members connected with hinged and fixed connections at connection nodes in each corner of the bay. Diagonal members along each face of the bay provide structural rigidity and are equipped with mid-length, self-locking hinges to allow the structure to collapse. Many other clever schemes for the articulated folding of repeating bay booms, or truss structures, have been arranged.
For example, U.S. Pat. No. 4,475,323, issuing to Schwartzberg et al., which is incorporated herein by reference, discloses a deployable box truss hoop. U.S. Pat. No. 4,557,097 to Mikulas, Jr. et al., which is also hereby incorporated by reference, discloses a sequentially deployable, maneuverable tetrahedral beam truss structure. U.S. Pat. No. 4,569,176 to Hedgepeth et al., hereby incorporated by reference, discloses a deployable lattice column having three sides and rigid diagonal members formed of rigid elements. U.S. Pat. No. 4,599,832 to Benton et al. (“Benton”), hereby incorporated by reference, discloses an extendible structure that can be collapsed to a shorter length and extended to a longer length. The extendible structure disclosed in Benton comprises a pair of station members interconnected by at least three longeron members. Each longeron member has two longeron elements that are pivoted together so they can fold toward one another or be aligned to form a column. Each longeron element is pivoted to a respective station member. Preloaded diagonal cable stays rigidify the structure when extended, being opposed by buckling springs (or Euler columns) that exert a radially outward resultant force in each bay at the folding point of each longeron member. The articulated lattice configuration disclosed in Benton was used to deploy the solar arrays on the international space station.
Another example of a repeating bay boom or truss structure comprised of collapsible bays formed from articulating members is provided in U.S. Pat. No. 4,677,803 to Mikulas, Jr. et al. (“Mikulas”), which is hereby incorporated by reference. The Mikulas patent discloses a deployable geodesic truss structure. The Mikulas geodesic truss structure includes a series of bays, each bay having sets of battens connected by longitudinal cross-members that give the bay its axial and torsional stiffness. The cross-members are hinged at their mid-point by a joint so that the cross-members are foldable for deployment or collapsing. Hinged longerons may also be provided to connect the sets of battens and to collapse for stowing with the rest of the truss structure. U.S. Pat. No. 5,267,424, issuing to Douglas, and which is hereby incorporated by reference, discloses a “bay” or, as referred to in the patent, a “module” for forming an articulated stowable and deployable mast. Further, U.S. Pat. No. 6,076,770 to Nygren et al., which is hereby incorporated by reference, discloses a folding truss that comprises a number of articulating column members.
Favored designs of articulated truss structures for space applications share traits of high performance in mass efficiency, low packaged volume, and reliability. Joint design is also important to performance of articulated truss structures since joints typically carry risks of reliability, increase the mass of the structure, and limit compaction.
An alternative lattice truss structure with joint-less longerons, and hence higher compaction and lower risk, is the coilable lattice truss boom. Numerous adaptations of this often employed structure exist. For example, U.S. Pat. No. 4,918,884 to Okazaki et al., which is hereby incorporated by reference, discloses an example of a coilable lattice truss that employs a plurality of radial spacers to define bays along a plurality of longerons arranged parallel to one another and attached to a pair of endplates. A pair of diagonal cords are stretched between adjacent radial spacers, between one of the end plates and the uppermost radial spacer and between the other of the end plates and the lowest radial spacer, respectively. A means is attached to one of the paired diagonal cords stretched between one of the end plates and the uppermost or lowest radial spacer to apply a predetermined tension to the diagonal cord. To collapse the truss structure, the longerons are elastically buckled between the radial spacers so as to coil the longerons between the endplates. The transforming of the longerons, and longitudinal position of the radial spacer located at one end of the structure, can be restrained by a means of applying overall axial tension while the structure is being deployed or collapsed. Other examples of coilable lattice truss booms are described in U.S. Pat. No. 3,486,279 to Webb for a deployable lattice column; U.S. Pat. No. 4,334,391 to Hedgepeth et al. for a redundant deployable lattice column; U.S. Pat. No. 4,532,742 to Miura for an extendible structure; and U.S. Pat. No. 5,094,046 to Preiswerk for a deployable mast.
Because the longerons in coilable lattice booms are highly strained when coiled for stowage, the material of choice for such longerons is typically a flexible glass fiber composite, such as an S2 glass fiber composite. As a result, in typical performance regimes, current coilable truss designs possess far in excess of a desirable amount of stowed strain energy, resulting in excessive push forces. This, in turn, requires the use of equipment sized to handle the resulting push force while the truss is in the stowed configuration, as well as when it is being deployed or collapsed. The required additional mass of the deployment mechanism to safely handle the push force of current coilable trusses adds parasitic mass and limits their overall mass efficiency.
Articulated and coilable lattice truss structures have been successful to date in providing low mass solutions to a wide array of lightly-loaded truss structures (relative to terrestrial structures) for use in space applications. But many potential space applications, including, for example, even more lightly loaded or “gossamer” applications and imaging mission applications requiring lightweight and stable structures, call for extendible structures having compaction, stability, and/or mass efficiency requirements that are outside the capabilities of existing structures or are not easily met by such structures. Accordingly, the ever increasingly challenging requirements for compaction, stability, and mass efficiency call for new generation extendible structure solutions.
High-performance graphite fiber composites potentially provide a huge gain in stiffness to weight capability over other available material options, such as flexible glass fiber composites, such as S2 glass fiber composites, and possess very low coefficients of thermal expansion. These are critical traits for future stable gossamer structures. Graphite fiber composite materials have limited applicability in known coilable lattice structures because graphite fiber composite materials have strain capabilities typically two to three times lower than glass fiber composite materials. Therefore, only much smaller, and hence weaker, longer rods can withstand the curvature encountered during stowage. The local buckling strength of a longeron is a function of the rod inertia, which is proportional to the diameter to the fourth power. This limits the utility of graphite composite longerons in currently practiced coilable lattice structure because the maximum diameter graphite longeron (approximately ⅓ that of an S2 glass fiber longeron) that can be used in known coilable structures would possess up to approximately 80 times less inertia. Even granting that a graphite rod is likely to be as much as 4 times stiffer in extensional modulus than a S2 rod, the buckling strength will still be 20 times lower than the heritage material (assuming equal column length).
Graphite fiber composite elements such as rods, and in larger structures, tubes, have been well utilized in articulated lattice structures in recent years. But, as always, the stacking of individual longeron elements restricts compaction capability because slenderness limits constrain the minimum realistic diameter of the longeron elements.
In recent years, numerous inflatable systems, which can use graphite fiber composites, have been under intense development in the hope that such systems would achieve a leap in mass and packaging efficiency, allowing ever larger systems to be packaged within the constraints of affordable launch systems. In practice, it has been difficult to achieve the structural efficiency of an articulated structure with an inflatable system due to mass overhead in non-structural systems such as: bladder materials, thermal barrier layers, node fittings, and inflation equipment and sequencing mechanisms. Inflatable systems are also plagued with structural inefficiencies inherent with the use of folding or rolling collapsed composite tubes. To allow the folding or rolling of collapsed composite tubes, the graphite material must be capable of withstanding high strain, requiring a reduction in fiber stiffness, fiber-to-matrix volume ratios, and/or the use of a woven fabric, which reduces the effective stiffness.
High performance tubular composite systems require composite tubes with maximum structural stiffness and high stability. Composite tubes achieve maximum structural efficiency when constructed from layered-fibers mostly oriented nearly axially to the lengthwise direction of the tube. The most stable composite tube lattice structure would be joined by bonding at composite nodes. However, such systems are not generally collapsible, although some have been proposed. One such proposed system is described in U.S. Pat. No. 6,321,503 to Warren. The mass efficiency of this system is high and the structure is stable, but the compaction ratio is poor. Allowing the tubes to be partially flattened, as described in U.S. Pat. No. 6,343,442 to Marks, increases the compaction, but it is still not satisfactory.
Inflatables, folded, and flattened lattice structures do not have precise kinematics and suffer from reduced stiffness and strength during deployment. Articulated lattice structures have precise kinematics that can be controlled by separate actuators and rate limiting devices. The reliability inherent in the heritage methods of deploying articulated lattice structures is also a key performance parameter. Reliability is another fundamental criteria in the creation of a desirable deployable structure for use in space applications, alongside mass efficiency, compact stowage performance, and stability.
A need, therefore, exists for deployable truss structures that improve on one or more of the above noted deficiencies of currently known deployable truss structures, yet maintain the reliable deployment characteristics of articulated and coilable lattice structures. Preferably, such truss structures would also improve on at least one of the attributes of mass efficiency, stowage volume, and thermal stability, and preferably all three. A need also exists for such structures that can make practical use of high-performance graphite fiber elements. A need further exists for column members that will enable improved deployable truss structures to be built.
An object of the present invention is to meet one or more of the foregoing needs.